Cascaid was appointed following a three-way pitch on the strength of its charity sector expertise, experience and "passionate understanding".
The agency will work closely with RSPB staff to ensure brand continuity and consistency, internally and externally.
The work will include addressing all major programmes and key activities including member recruitment and supporter development, campaigning and lobbying for change, events, face-to-face and onsite materials.
Cascaid's first work will involve the creation of messages for direct membership recruitment that build understanding of what the RSPB delivers for conservation.
It is expected to begin with a focus on "personal connections" with nature and then to explore the bigger picture of global issues.
Rob George, head of marketing insight and systems at the RSPB, said: "Over 80% of the UK population is aware of the RSPB. However, many people don't know the extent of what we do, especially around the bigger environmental issues, such as climate change, or our international work and campaigning."
Roger Lawson, strategy and planning director of Cascaid, said: "Emotive engagement is the key, we want people to truly understand how relevant the RSPB's work is both to both the health of the planet and to them personally."